1. Field of the Invention
The present invention relates to an opening and closing apparatus of a manhole cover provided on a manhole of an multipurpose underground conduit storing water and sewage pipes, electric cables, telephone cables, and the like collectively, and the like.
2. Description of the Related Art
As a manhole cover provided on a manhole of an multipurpose underground conduit, there are a concrete cover and a cast-iron cover, where the cast-iron cover has been generally used as a manhole cover provided on a place where opening and closing operations are often performed or a place where an area of an opening portion is large. However, the gross weight of the cast-iron cover depends on a shape or a size thereof, but it reaches 600 to 800 kg, so that it is difficult to utilize human power to open and close the cover, a heavy equipment such as a track crane or an exclusive opening and closing apparatus must be used for opening and closing the cover, and such an accident as dropping the cover into a manhole may happen due to erroneous opening and closing operation.
In order to solve such conventional problems, the present inventor has developed such an opening and closing apparatus of a manhole cover as shown in FIG. 1 to FIG. 10, FIG. 13, and FIG. 14 (Japanese Patent No. 2914633 and Japanese Patent No. 3299729). The opening and closing apparatus, as shown in these figures, is an opening and closing apparatus of a manhole cover in which a manhole cover 3 is rotatably attached on an opening portion 1A of a doughnut-shaped supporting frame 1 via a rotational joining portion 2, a hydraulic jack 7 in which a plunger pump 5 is driven by an operation lever 4 to raise a ram 6 and an oil returning valve 34 is released, to lower the ram 6 is provided near the rotational joining portion 2 of the supporting frame 1 rotatably by a mounting member 8 provided on a lower end portion of the hydraulic jack 7, and the hydraulic jack 7 and an engagement member 9 fixed attachably to and detachably from the manhole cover 3 are coupled to each other by a pulling member 10.    Patent Literature 1: Japanese Patent No. 2914633    Patent Literature 2: Japanese Patent No. 3299729
In the opening and closing apparatus of the manhole cover thus configured, there is such an advantage that even a heavy cover can be easily opened and closed by a worker alone in a short time. In the opening and closing apparatus, however, there are structural problems especially on an apparatus for rotating the manhole cover 3 to open and close the opening portion 1A of the supporting frame 1, that is, the hydraulic jack 7 which is a driving source for performing opening and closing. However, an opening and closing apparatus of a manhole cover that is further excellent in safety could have been provided by solving these problems.
An oil returning valve 34, a screw valve chamber 35, an outlet portion 36, and a return port 37 is configured as shown in FIG. 13 and FIG. 14, that is, it is configured to change from a state shown in FIG. 13 in which a screw valve 38 screwed so as to be capable of advancing and retreating within the screw valve chamber 35 has advanced and the oil returning valve 34 has closed to a state shown in FIG. 14 in which the oil returning valve 34 is released by rotating the screw valve 38 to retreat the same so that the oil return path runs on in one line manner.
Specifically, the oil returning valve 34 is configured to be closed by supporting a ball 34A at a distal end of the screw valve 38 attached by screwing such that the ball 34A can move along the inside of the screw valve chamber 35 to come in pressure contact with a seat surface 34B and to be released or opened by loosening (retreating) the screw valve 38 to separate the ball 34A from the seat surface 34B.
An operation for releasing the oil returning valve 34 is required when, after work in the manhole has been finished, the manhole cover 3 is going to be closed from a state shown by solid line in FIG. 9, that is, a state shown in FIG. 10 in which the manhole cover 3 has been fully opened. That is, the screw valve 38 is loosened from a state shown in FIG. 13 and moved leftward in FIG. 13, and after the oil returning valve 34 is released as shown in FIG. 14, the manhole cover 3 is pushed out rightward in FIG. 9 and FIG. 10 in a state shown by solid line in FIG. 9 and shown in FIG. 10 by hand, and rotated about the rotational joining portion 2 to be put into a state shown by dashed line in FIG. 9.
When the oil returning valve 34 has been released and the manhole cover 3 has been moved to a state shown by dashed line in FIG. 9, the manhole cover 3 is subjected to force in a direction (rightward) in which the manhole cover 3 falls rightward due to its own weight. As a result, the manhole cover 3 rotates about the rotational joining portion 2 in a closing direction shown in FIG. 8 from a state shown by dashed line in FIG. 9, and further rotates at once into a state shown in FIG. 7 in which the manhole cover 3 fully closes the opening portion 1A of the supporting frame 1.
Then, in order to prevent hydraulic oil from rapidly returning through the oil returning valve 34, the screw valve chamber 35, the outlet portion 36, and the return port 37, a plurality of structures in which a hole diameter of the return port 37 was made further smaller than that of a conventional return port shown in FIG. 13 and FIG. 14 were manufactured, and an experiment on a relationship between the hole diameter of the return port 37 and a speed at a time when the manhole cover 3 was closed was repeated.
It was expected that resistance of oil flowing inside the return port 37 was increased by making the hole diameter of the return port 37 small so that a movement flow rate of the hydraulic oil slowed down and the manhole cover 3 slowly closed. The manhole cover 3 gently rotated at the beginning of the experiment due to that the hole diameter of the return port 37 was made small, but the return port 37 was clogged when used for a long time due to oil degradation, dust, contamination, or the like, which caused such a problem that the hydraulic jack 7 did not work.
The present invention has been made to solve such a conventional problem as described above, and an object thereof is to provide an apparatus in which a detour orifice for adjusting a returning flow rate of hydraulic oil is provided in an oil returning path through which the hydraulic oil returns from a hydraulic pressure chamber on the side of the ram to a tank so that a returning speed of the hydraulic oil returning to the tank from the pressure oil chamber on the side of the ram is adjusted to be slowed down as much as possible, so that a manhole cover slowly rotates and an opening portion of a supporting frame can close gently.